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Conduction-band edge dependence of carbon-coated hematite stimulated extracellular electron transfer of Shewanella oneidensis in bioelectrochemical systems
Bioelectrochemistry 102 (2015) 29–34
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Bioelectrochemistry journal homepage: www.elsevier.com/locate/bioelechem
Conduction-band edge dependence of carbon-coated hematite stimulated extracellular electron transfer of Shewanella oneidensis in bioelectrochemical systems Shungui Zhou, Jiahuan Tang, Yong Yuan ⁎ Guangdong Institute of Eco-environmental and Soil Sciences, 808 Tianyuan Road, Guangzhou City, Guangdong Province, PR China
a r t i c l e
i n f o
Article history: Received 23 May 2014 Received in revised form 12 November 2014 Accepted 17 November 2014 Available online 29 November 2014 Keywords: Carbon-coated hematite Extracellular electron transfer Conduction-band edge Semiconductor Bioelectrochemical system
a b s t r a c t Bacteria-based bioelectrochemical systems (BESs) are promising technologies used for alternative energy generation, wastewater treatment, and environmental monitoring. However, their practical application is limited by the bioelectrode performance, mainly resulting from low extracellular electron transfer (EET) efficiency. In this study, a carbon-coated hematite (C/Hematite) electrode was successfully obtained by a green and solventfree route, that is, heat treatment in an oxygen-rich environment using solid ferrocene as the precursor. The as-prepared C/Hematite electrode was evaluated as a high-performance electrode material in a Shewanella oneidensis-inoculated BES. The maximum biocurrent density of the Shewanella-attached C/Hematite electrode reached 0.22 ± 0.01 mA cm−2, which is nearly 6-times higher than that of a bare carbon cloth (CC) electrode (0.036 ± 0.005 mA cm−2). Electrochemical measurements revealed that the enhanced conductivity and better energy matching between the outer membrane c-type cytochromes of S. oneidensis and the electrode contributed to the improved EET efficiency. The results of this study demonstrated that the semiconductive properties of iron oxides play important roles for the involved bacterial extracellular respiration activities. © 2014 Elsevier B.V. All rights reserved.
1. Introduction It has been discovered that the ability of microbes to electrically interact with conductive solids is harnessed in bioelectrochemical systems (BESs), which have shown great potential for applications in alternative energy generation, wastewater treatment, and environmental monitoring [1–3]. A few microbes known as exoelectrogens, such as Shewanella and Geobacter species, have been identified as possessing this function [4,5]. It is hypothesized that such electron transfer occurs through conductive pili and/or outer membrane (OM) c-Cyts, or through indirect electron transfer via electroactive metabolites [6]. Although this is a promising biotechnology, the practical application of BESs is currently limited by the bioelectrode performance, which mainly results from low efficiency of extracellular electron transfer (EET), low biomass density on the electrode surface, limited mass transfer, and so on [7,8]. A facile approach to increase the performance of the BES is often to modify conventional carbon electrodes for enhancing the actual accessible electrode surface area for exoelectrogens, improving electrode biocompatibility, and facilitating the EET between the electrode and bacteria [9–11]. Carbon materials with unique electrical and structural
⁎ Corresponding author. E-mail address: [email protected] (Y. Yuan).
http://dx.doi.org/10.1016/j.bioelechem.2014.11.005 1567-5394/© 2014 Elsevier B.V. All rights reserved.
properties, e.g., carbon nanoparticles, carbon nanotubes, and graphene, were found to be efficient modifiers for such functions [12–14]. In addition to carbon-based materials, iron oxides have also been employed to modify carbon electrodes to increase the EET in BESs because of their high biocompatibility and great specific affinity to the bacterial OM cCyts [15–17]. Kim et al. previously described that the use of an iron oxide-coated electrode enhanced power generation by ca. 4 times in a BES inoculated with a mixed culture [15]. However, the fabrication of an iron(III) oxide-based electrode typically involves complex chemical synthesis of iron oxides and a modification process, such as layer-bylayer self-assembly or roll–press techniques [16,17]. In this respect, it is highly desired to exploit facile approaches to fabricate efficient iron oxide electrode for targeting high EET. In the present work, we employed a facile strategy to fabricate a hematite (α-Fe2O3)-coated carbon cloth electrode by simply pyrolyzing solid ferrocene under ambient pressure. Because the hematite was directly prepared by the pyrolysis of ferrocene, the carbon in ferrocene was maintained in the resulting hematite, producing a carbon-coated hematite that displayed improved properties for directing the EET of Shewanella oneidensis compared to unmodified hematite. In addition, carbon-coated hematite (C/Hematite) was directly achieved from a solid chemical compound, representing a green and solvent-free strategy. We demonstrated that lab-scale Shewanella-inoculated BESs equipped with this C/Hematite electrode significantly outperformed those with unmodified or conventional hematite-coated electrodes.
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2. Materials and methods 2.1. Preparation of C/Hematite electrode The preparation of C/Hematite-coated carbon cloth is illustrated in Fig. S1A. Ferrocene (0.2 g, 98%, Sigma-Aldrich) was added to a crucible (30 mL) covered by a carbon cloth (Type A, 5.0 cm in diameter). The ferrocene-containing crucible was then heated to 450 °C. The loading of C/Hematite on the carbon cloth was ca. 1.0 mg cm− 2, which was assessed by evaluating the weight of the carbon cloth (CC) before and after the hematite coating. For comparison, the same amount of chemically synthesized hematite (denoted as Hematite) and hematite mixture with carbon powder (denoted as C–Hematite) was coated onto the surface of CC, as suggested by Lowy et al. [18]. The ratio of carbon powder to hematite was 1:5.7, which was the same as that in C/Hematite. Hematite was chemically synthesized following the method of Yen et al. [19]. 2.2. Formation of S. oneidensis biofilms on various electrodes S. oneidensis MR-1 bacterial culture suspension (1.0 mL) was firstly inoculated into 50 mL fresh LB broth (peptone, 10.0 g/L; yeast extract, 5.0 g/L; NaCl 1.0 g/L, pH 7.0), and incubated at 30 °C till the optical density (OD600) of the cell culture reached ~2.0. Then, the cells were harvested by centrifugation and washed 3 times with N2-purged phosphate buffer (Na2HPO4·12H2O, 11.4 g/L; NaH2PO4·2H2O, 2.8 g/L; KCl, 0.13 g/L; NH4Cl, 0.31 g/L, pH 7.0). The cell pellets were subsequently re-suspended in 50 mL electrolyte (phosphate buffer supplemented with 10 mM sodium lactate). The cell suspension was transferred into bioelectrochemical systems (BESs), and all working electrodes were poised at − 0.1 V vs. saturated calomel electrode (SCE). The BES (28 mL in volume) had a configuration as shown in Fig. S2. Bioelectrochemical experiments were carried out under multi-channel potentiostatic control (CHI1000C, China) utilizing a three-electrode arrangement with the as-prepared hematite electrodes as the working electrode (1.0 cm in diameter), a SCE reference electrode and a Pt net counter electrode. All tests were conducted in triplicate, and the mean values and typical curves are presented. 2.3. Electrochemical measurements Cyclic voltammetric (CV) measurements were performed with the biofilm-attached electrode as the working electrode under turnover conditions. Electrochemical impedance spectroscopy (EIS) was performed using an Autolab PGSTAT 302N potentiostat (Utrecht, the Netherlands) with NOVA software. The EIS of the biofilm-attached electrode was measured at the open circuit potential and in the frequency range of 1 × 106 to 0.5 Hz with a sinusoidal perturbation amplitude of 5 mV. The obtained Nyquist plots and circle fitting NOVA software were used to analyze the resistance of the bioelectrodes. Photoelectrochemical measurements of the hematites were conducted using a 300 W Xe lamp equipped with an interference filter with a band width of 10 nm as a light source, as suggested by Nakamura et al. [20]. Mott–Schottky plots were generated using the capacitance values derived from the EIS data fitting, from which the flat band potential and charge-carrier concentration were extracted as previously suggested [21]. The EIS measurements were also performed by a three-electrode configuration using Autolab PGSTAT as described above. A sinusoidal voltage perturbation with an amplitude of 5 mV and a frequency ranging from 1 × 105 to 1 Hz was superimposed on the bias voltage. The impedance was measured at bias voltages from 1.1 to 2.1 V vs. reversible hydrogen electrode (RHE). The RHE was converted from the SCE with the followed formulation:
ERHE ¼ ESCE þ 1:051 V ð1 M KOHÞ:
ð1Þ
All EIS measurements were performed in the dark. The Nyquist plot obtained from the EIS measurements was simulated using the NOVA software. The flat band potential and carrier density at the hematite/electrolyte interface can be obtained by the Mott–Schottky equation: 2
1=C ¼ ð2=e0 εε0 Nd Þ½ðE−EFB Þ−kT=e0
ð2Þ
where C is the specific capacitance (F/cm2), e0 is the electron charge, ε is the dielectric constant of hematite, ε0 is the permittivity of vacuum, Nd is the carrier density, E is the electrode applied potential, EFB is the flatband potential, and kT/e0 is a temperature-dependent correction term. 2.4. Analytical techniques The X-ray diffraction (XRD) patterns of hematites were recorded on an Empyrean diffractometer at room temperature, operating at 40 kV and 40 mA, using a Cu Kα radiation (λ = 0.15418 nm). The X-ray photoelectron spectra (XPS) were recorded on an ESCALAB 250 (Thermo Fisher Scientific) using monochromatic Al Ka X-rays (1486.69 eV) with an X-ray power of 150 W. Scanning electron microscopy (SEM) was used to confirm the morphologies of the prepared electrode and the biofilms as described previously [11]. Ferrous and ferric irons were monitored spectrophotometrically by the ferrozine assay according to previous methods [22]. All measurements were operated in duplicate and the typical image and curve were presented. 3. Results and discussion 3.1. Preparation and characterization of the C/Hematite-coated electrode Illustration of the experimental setup for the synthesis of the C/Hematite-coated electrode is found in Fig. S1A. The principle of the synthesis is that ferrocene was first melted and boiled in a crucible, then sublimed and decomposed to form hematite upon oxidation of oxygen as the temperature increased. As a result, a layer of red-colored film can be observed on the substrates covering the crucible. Using this method, Deng et al. has previously proposed the manipulation of
A
B
1 μm
1 μm
D
C
1 μm
1 μm
Fig. 1. SEM images of the blank CC (A), C/Hematite covered CC (B), Hematite covered CC (C) and C–Hematite covered CC (D, inset: SEM image in low resolution shows the presence of carbon particles as indicated by the arrows).
S. Zhou et al. / Bioelectrochemistry 102 (2015) 29–34
C/Hematite-covered conductive glasses, from which a high photocurrent was achieved for solar water splitting [23]. The enhanced photocurrent was attributed to the modified electronic structure of the C/Hematite with the existence of oxygen vacancies because of the presence of the carbon layer. However, it was noted that the hematite coating on the glasses was not homogeneous because of the inconsistent oxygen exposure. Herein, we covered a piece of carbon cloth (CC) on a crucible with a homogeneous layer of red-colored film (Fig. S1A). Carbon cloth is a type of carbon fiber fabric in a weave pattern with plenty of homogeneously distributed holes (Fig. S1), which ensures homogeneous delivery of oxygen for reaction with ferrocene vapor. As shown in Fig. 1A–B and Fig. S1B–E, the SEM images provide further evidence that C/Hematite was homogenously coated onto the CC. This method resulted in C/Hematite nanoparticles, which differ from the chemically synthesized rod-shaped hematite (Fig. 1C). The image of C–Hematite clearly shows the combination of hematite with carbon particles (Fig. 1D). Elemental mapping and Energy-dispersive X-ray spectroscopy (EDS) revealed that Fe, O and C were the main elements in the resultant hematite in which C was distributed into the hematite (Fig. 2A and B). XRD patterns of the resultant products from ferrocene exhibited similar signatures to those of hematite (Fig. 2C), indicating the successful synthesis of hematite from ferrocene. The observed reflections were indexed according to the rhombohedral structure and were consistent with the JCPDS data (card no. 33-0664). The observed sharp, intense reflections confirm the highly crystalline nature of the powders synthesized by heat treatment. XPS also revealed that Fe, O and C were the main elements in the C/Hematite (Fig. S3). The C1s XPS spectra showed a signal at approximately 284.8 eV, indicating the presence of C–C bonds in the C/Hematite. The binding energies for Fe2p3/2 and Fe2p1/2 are 711.2 and 724.6 eV, respectively, which are in excellent agreement with the reported value for Fe2O3 [24]. The O1s XPS spectra showed a signal at approximately 530.1 eV, which was also consistent with the reported value for Fe2O3 (530.1 eV for the original α-Fe2O3 and 530.3 eV for doped α-Fe2O3) [24]. The shoulder at higher binding energy (531.8 eV) could be attributed to O–C bonds.
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3.2. Biocurrent generation enhanced by C/Hematite To evaluate the effect of C/Hematite on the interfacial EET of S. oneidensis, a BES was constructed with the fabricated electrodes as the working electrodes. S. oneidensis cells were first inoculated into the BESs, and all working electrodes were poised at − 0.1 V vs. SCE (Fig. S2, Supporting information). As shown in Fig. 3A, the microbial electric currents gradually increased with time, reflecting lactate oxidation by cells and electron transfer from the cells to the electrodes. The current from the bare CC electrode reached a maximum current density of 0.036 ± 0.005 mA cm−2, and a similar level using carbon papers was previously reported [25,26]. When the CC was coated with chemically synthesized hematite, the current reached a maximum of 0.060 ± 0.007 mA cm− 2. The current density further increased, reaching a maximum of 0.09 ± 0.006 mA cm−2 when the CC was coated with the C–Hematite mixture. Notably, after the C/Hematite coating, the current density dramatically increased to a maximum of 0.22 ± 0.01 mA cm−2, representing a nearly 6-fold increase compared to that of a bare CC electrode. A coulombic efficiency (CE) of 34 ± 3% was obtained in the BES with the C/Hematite electrode, which is higher than that of C–Hematite (19 ± 2%), Hematite (13 ± 2%), and CC (7 ± 1%) electrodes. The higher current densities and CE from the hematitecoated electrode are partially attributed to the higher affinity of the hematite-coated electrodes for bacterial cells compared to that of the bare CC electrode. The strengthened interaction between bacteria and C/hematite made the electrode attractive enough for the bacteria to transfer the electrons, which weakened alternative processes that did not contribute to current production and subsequently increased the CE. As shown in Fig. S4, more bacterial cells were observed on the hematite-coated CC than those on the bare CC. As for S. oneidensis MR-1, the outer membrane c-type cytochrome exhibited a specific binding to iron oxides [27]. This interaction not only enabled bacteria to insert electrons into the iron oxides [28] but also improved the bacteria/solid interfacial electron transfer [29]. Fig. S5A shows that the stable current output was achieved from the BES with the bacteriaattached C/Hematite electrode under the batch mode operation,
A
10 μm
Fig. 2. (A) Elemental mapping of C/Hematite: carbon (red), oxygen (yellow), and iron (green); (B) EDS analysis of C/Hematite; (C) XRD analysis of Hematite (red line) and C/Hematite (black line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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3.3. Electrochemical characteristics of the bioelectrodes The extracellular electron transfer capability of the S. oneidensis cells was further examined using CV scans. Fig. 3B shows the whole-cell CV recorded from various cell-attached bioelectrodes. The CV data of the bioelectrodes showed a pair of well-defined redox peaks in the potential range between −0.7 and 0 V vs. SCE, in which a midpoint potential (Em) was calculated according to Em = (Epc + Epa) / 2. An Em value of −0.160 V vs. SCE was obtained for the S. oneidensis cells attached to the CC electrode. This value is in accordance with the previously reported value for OM c-Cyts isolated from Shewanella and also Shewanella cells [32,33]. These results suggested that S. oneidensis cells expressed c-type cytochromes that are essential to the EET and could interact elecrochemically with external electrodes. To further understand the high current generation by the C/Hematite-coated CC, AC impedance was performed. Fig. 3C and D shows the typical Nyquist impedance spectra of bioelectrodes with the equivalent circuit model, where the independent axis is the real impedance (Z′ (ohm)) and the dependent axis is the imaginary impedance (−Z″ (ohm)). The parameter Rs represents the solution resistance, R1 is the resistance of the bioelectrodes, and R2 is the charge transfer resistance [34]. Constant phase elements (CPE1 and CPE2) are used to model non-ideal capacitors of the biofilm and double layer [34], respectively, defined as ZCPE = 1 / [Q(iω)n], where Q has a dimension of F sn − 1, and n is a dimensional parameter accounting for non-ideal behavior [35]. The fitting results are listed in Table 1. The Rs values for the BESs with various bioelectrodes were nearly identical because of the use of the same electrolyte. However, the different R1 values were observed due to the varied biomass loading and coating materials. The lowest R1 value was obtained from the C/Hematite-coated bioelectrode, likely resulting from the enhanced interaction between S. oneidensis cells and C/Hematite-coated electrode. The higher Q1 value of the C/Hematite-coated bioelectrode suggested the presence of the more active biofilm due to its properties as a charge storage body [36], and the higher Q2 value demonstrated the higher double layer capacitance of the C/Hematite than those of the other bioelectrodes [37]. In addition, the R2 values of the bioelectrodes were significantly varied. The R2 can serve as a measure of the electrocatalytic activity of the electrode because the R2 value is inversely proportional to the exchange current density of the reaction that takes place at each electrode [38]. An R2 of 14 Ω was achieved from the C/Hematite electrode, which is lower than that of the C–Hematite, Hematite and CC electrodes at 22, 36, and 60 Ω, respectively. The lowest R2 suggested the fastest electron transfer rate of the C/Hematite-coated bioelectrode. Fig. 3. Electrochemical characterizations of various electrodes (black line — C/Hematite, red line — C–Hematite, green line — Hematite, and blue line — CC): (A) Current generation by S. oneidensis with different electrodes poised at −0.1 V vs. SCE (the arrow indicates the addition of sodium lactate); (B) CVs of biofilm-based electrodes in the presence of substrate, scan rate = 10 mV s−1; (C) Nyquist curves of the EIS tests: black squares — C/ Hematite, red dots — C–Hematite, green triangles — Hematite, blue rhombs — CC, magenta lines — fitting curves (inset: the equivalent electrical circuit and the enlarged highfrequency region). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
suggesting the high stability of the C/Hematite electrode. It is well known that iron ions could be released from hematite in the presence of iron reducing bacteria [30], which positively affects the electron transfer rate of S. oneidensis [31]. As shown in Fig. S5B, release of iron ions from C/Hematite was also observed; however, only a very small amount of iron (total iron concentration of ~ 5 μM) was detectable in the electrolyte. With this small amount of iron, the current output was slightly affected. As shown in Fig. S5C, the maximum current density from the biofilm-attached CC electrode was 0.035 mA cm− 2, which slightly increased to 0.038 mA cm − 2 after the addition of 5 μM exogenous ferrous ions. The results suggested that the improved current generation from the C/Hematite electrode mainly resulted from the direct electron transfer facilitated by C/Hematite rather than the indirect electron transfer mediated by the released iron.
3.4. Conduction-band edge dependence of hematite stimulated EET To understand the role of carbon-coating on the electronic properties of hematite in electrolyte solution, EIS was conducted in the dark to determine the capacitance of hematites. The carrier density and flatband potential (EFB) at the hematite/electrolyte interface were evaluated by the Mott–Schottky equation. By analyzing the Mott–Schottky plots (Fig. 4), the EFB potentials were determined to be 0.37, 0.39, and 0.44 V vs. RHE for hematite, C–Hematite, and C/Hematite, respectively. The EFB potentials were consistent with the values previously reported for other hematites [21]. The charge-carrier concentrations were calculated to be 1.7 × 1019, 2.1 × 1019, and 3.7 × 1019 cm−3 for hematite,
Table 1 EIS fitting results for various bioelectrodes. Condition
Rs (Ω)
R1 (Ω)
Q1 × 10−4 (F sn − 1)
n1
Q2 × 10−6 (F sn − 1)
n2
R2 (Ω)
C/Hematite C–Hematite Hematite CC
6 6 7 5
189 261 387 545
9.3 5.5 3.4 2.4
0.80 0.80 0.90 0.80
4.69 2.89 2.36 1.88
0.80 0.80 0.79 0.70
14 22 36 60
S. Zhou et al. / Bioelectrochemistry 102 (2015) 29–34
Fig. 4. Mott–Schottky plots of hematites collected at a scan rate of 10 mV/s (in 1 M KOH electrolyte at a frequency of 10 kHz), from which the flat band potential and charge-carrier concentration were extracted: C/Hematite (squares), C–Hematite (dots), and Hematite (triangles).
C–Hematite, and C/Hematite, respectively. With a higher EFB value and a larger charge-carrier concentration, the C/Hematite-coated electrode obtained better bioelectricity generation in the BES inoculated with S. oneidensis. Low conductivity is one of the major factors limiting the use of hematite in energy conversion applications, which is related to the charge-carrier concentrations and/or carrier mobility [39]. In comparison to unmodified hematite, the higher charge-carrier concentration demonstrated that higher electrical conductivity of C–Hematite and C/Hematite could be obtained, which benefit the hematitemediated electron transfer on the hematite–microbe interface. It is well known that S. oneidensis MR-1 applies a sophisticated protein network to transfer electrons from the cytoplasmic membrane across the periplasmic space and through the outer membrane to the extracellular cell surface, where a putative metal reductase stimulates terminal electron transfer directly to solid acceptors [40,41]. The terminal metal reductase is confirmed to be composed of outer membrane, decaheme c-type cytochromes. In particular, two of these OM c-Cyts, MtrC (locus tag SO1778; also known as OmcB) and OmcA (locus tag SO1779), have been shown to play an important role in direct electron transfer to iron oxides in the case of S. oneidensis [27]. However, the matched energy level of terminal OM c-Cyts with the conductionband edge (ECB) of iron oxides was a requirement for the greatly stimulated EET [20]. Herein, the ECB values of the resulting iron oxides were evaluated to be approximately − 0.22, − 0.19, and − 0.17 V (vs. SCE) for hematite, C–Hematite and C/Hematite, respectively, from the onset potential of the anodic photocurrents (Fig. S6). It was found that the OM c-Cyts of Shewanella cells had a midpoint potential (Em = −0.160 V vs. SCE) close to the conduction-band edge of C/Hematite (ECB = −0.17 V vs. SCE), whereas a large energy separation existed for Hematite (ECB = − 0.22 V vs. SCE) and C–Hematite
Scheme 1. Conduction-band energy-level (ECB) diagram of the hematites with respect to the mid-point potential (Em) of outer-membrane c-type cytochromes for live S. oneidensis MR-1 cells.
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(ECB = −0.19 V vs. SCE) (Scheme 1). The better matched energy levels of the OM c-Cyts with the ECB of C/Hematite could contribute to the higher current generation of the C/Hematite-coated electrode compared to other hematite-coated electrodes. Indeed, several methods have been verified to design the electrochemical properties of the hematite for achieving better physical and chemical activities [35,42]. One effective method is to adjust the component of the hematite by using doping strategies [35]. As previously reported [43], the incorporation of “impurities” into hematite has two main effects: (i) the electronic band edges are modified; and (ii) the incorporation of some “impurities” increases the unit cell volume, which may also affect the electrical conductivity by changing the hopping probabilities. In this respect, the shifting of the ECB of the C/Hematite could be attributed to the incorporation of carbon into the hematite. However, improved electron transfer can also be achieved by dispersing fine particles of iron oxide onto the surface of the conductor [17,44]. For the same reason, carbon particles acted as the conductor for communicating the hematite colloids in the C–Hematite, leading to the shifted ECB and the accelerated EET compared to the non-modified hematite. 4. Conclusion In this study, carbon-coated hematite was proven to be an effective modification for high-performance electrodes with high biocurrent generation. The substantial improvement was attributed to the advantages of carbon–hematite coupling, which changed the electrical properties of hematite for electron transfer, resulting in an improved interaction between the microbial cells and the electrodes. Because the C/Hematite can be readily decorated onto electrode substrates through an evaporation method, C/Hematite may be a promising material for constructing highly efficient electrodes for BESs. Acknowledgments This study was supported jointly by the National Natural Science Foundation of China (Nos. 21277035 and 4122006). Appendix A. Supplementary data SEM and XPS analysis of electrodes and electrochemical properties of all hematite-coated electrodes (Fig. S1–6). Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j. bioelechem.2014.11.005. References [1] B.E. Logan, B. Hamelers, R. Rozendal, U. Schröder, J. Keller, S. Freguia, P. Aelterman, W. Verstraete, K. Rabaey, Microbial fuel cells: methodology and technology, Environ. Sci. Technol. 40 (2006) 5181–5192. [2] W.W. Li, H.Q. Yu, Z. He, Towards sustainable wastewater treatment by using microbial fuel cells-centered technologies, Energy Environ. Sci. 7 (2014) 911–924. [3] D.R. Lovley, Live wires: direct extracellular electron exchange for bioenergy and the bioremediation of energy-related contamination, Energy Environ. Sci. 4 (2011) 4896–4906. [4] B.H. Kim, H.J. Kim, M.S. Hyun, D.H. Park, Direct electrode reaction of Fe (III)-reducing bacterium, Shewanella putrefaciens, J. Microbiol. Biotechnol. 9 (1999) 127–131. [5] D.R. Bond, D.E. Holmes, L.M. Tender, D.R. Lovley, Electrode-reducing microorganisms that harvest energy from marine sediments, Science 295 (2002) 483–485. [6] K. Rabaey, N. Boon, M. Höfte, W. Verstraete, Microbial phenazine production enhances electron transfer in biofuel cells, Environ. Sci. Technol. 39 (2005) 3401–3408. [7] A.P. Borole, G. Reguera, B. Ringeisen, Z.W. Wang, Y.J. Feng, B.H. Kim, Electroactive biofilms: current status and future research needs, Energy Environ. Sci. 4 (2011) 4813–4834. [8] L. Peng, S.J. You, J.Y. Wang, Carbon nanotubes as electrode modifier promoting direct electron transfer from Shewanella oneidensis, Biosens. Bioelectron. 25 (2010) 1248–1251. [9] Y. Qiao, S.J. Bao, C.M. Li, X.Q. Cui, Z.S. Lu, J. Guo, Nanostructured polyaniline/titanium dioxide composite anode for microbial fuel cells, ACS Nano 2 (2007) 113–119. [10] X. Xie, L. Hu, M. Pasta, G.F. Wells, D. Kong, C.S. Criddle, Y. Cui, Three-dimensional carbon nanotube-textile anode for high-performance microbial fuel cells, Nano Lett. 11 (2010) 291–296.
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Conduction-band edge dependence of carbon-coated hematite stimulated extracellular electron transfer of Shewanella oneidensis in bioelectrochemical systems Shungui Zhou, Jiahuan Tang, Yong Yuan ⁎ Guangdong Institute of Eco-environmental and Soil Sciences, 808 Tianyuan Road, Guangzhou City, Guangdong Province, PR China
a r t i c l e
i n f o
Article history: Received 23 May 2014 Received in revised form 12 November 2014 Accepted 17 November 2014 Available online 29 November 2014 Keywords: Carbon-coated hematite Extracellular electron transfer Conduction-band edge Semiconductor Bioelectrochemical system
a b s t r a c t Bacteria-based bioelectrochemical systems (BESs) are promising technologies used for alternative energy generation, wastewater treatment, and environmental monitoring. However, their practical application is limited by the bioelectrode performance, mainly resulting from low extracellular electron transfer (EET) efficiency. In this study, a carbon-coated hematite (C/Hematite) electrode was successfully obtained by a green and solventfree route, that is, heat treatment in an oxygen-rich environment using solid ferrocene as the precursor. The as-prepared C/Hematite electrode was evaluated as a high-performance electrode material in a Shewanella oneidensis-inoculated BES. The maximum biocurrent density of the Shewanella-attached C/Hematite electrode reached 0.22 ± 0.01 mA cm−2, which is nearly 6-times higher than that of a bare carbon cloth (CC) electrode (0.036 ± 0.005 mA cm−2). Electrochemical measurements revealed that the enhanced conductivity and better energy matching between the outer membrane c-type cytochromes of S. oneidensis and the electrode contributed to the improved EET efficiency. The results of this study demonstrated that the semiconductive properties of iron oxides play important roles for the involved bacterial extracellular respiration activities. © 2014 Elsevier B.V. All rights reserved.
1. Introduction It has been discovered that the ability of microbes to electrically interact with conductive solids is harnessed in bioelectrochemical systems (BESs), which have shown great potential for applications in alternative energy generation, wastewater treatment, and environmental monitoring [1–3]. A few microbes known as exoelectrogens, such as Shewanella and Geobacter species, have been identified as possessing this function [4,5]. It is hypothesized that such electron transfer occurs through conductive pili and/or outer membrane (OM) c-Cyts, or through indirect electron transfer via electroactive metabolites [6]. Although this is a promising biotechnology, the practical application of BESs is currently limited by the bioelectrode performance, which mainly results from low efficiency of extracellular electron transfer (EET), low biomass density on the electrode surface, limited mass transfer, and so on [7,8]. A facile approach to increase the performance of the BES is often to modify conventional carbon electrodes for enhancing the actual accessible electrode surface area for exoelectrogens, improving electrode biocompatibility, and facilitating the EET between the electrode and bacteria [9–11]. Carbon materials with unique electrical and structural
⁎ Corresponding author. E-mail address: [email protected] (Y. Yuan).
http://dx.doi.org/10.1016/j.bioelechem.2014.11.005 1567-5394/© 2014 Elsevier B.V. All rights reserved.
properties, e.g., carbon nanoparticles, carbon nanotubes, and graphene, were found to be efficient modifiers for such functions [12–14]. In addition to carbon-based materials, iron oxides have also been employed to modify carbon electrodes to increase the EET in BESs because of their high biocompatibility and great specific affinity to the bacterial OM cCyts [15–17]. Kim et al. previously described that the use of an iron oxide-coated electrode enhanced power generation by ca. 4 times in a BES inoculated with a mixed culture [15]. However, the fabrication of an iron(III) oxide-based electrode typically involves complex chemical synthesis of iron oxides and a modification process, such as layer-bylayer self-assembly or roll–press techniques [16,17]. In this respect, it is highly desired to exploit facile approaches to fabricate efficient iron oxide electrode for targeting high EET. In the present work, we employed a facile strategy to fabricate a hematite (α-Fe2O3)-coated carbon cloth electrode by simply pyrolyzing solid ferrocene under ambient pressure. Because the hematite was directly prepared by the pyrolysis of ferrocene, the carbon in ferrocene was maintained in the resulting hematite, producing a carbon-coated hematite that displayed improved properties for directing the EET of Shewanella oneidensis compared to unmodified hematite. In addition, carbon-coated hematite (C/Hematite) was directly achieved from a solid chemical compound, representing a green and solvent-free strategy. We demonstrated that lab-scale Shewanella-inoculated BESs equipped with this C/Hematite electrode significantly outperformed those with unmodified or conventional hematite-coated electrodes.
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2. Materials and methods 2.1. Preparation of C/Hematite electrode The preparation of C/Hematite-coated carbon cloth is illustrated in Fig. S1A. Ferrocene (0.2 g, 98%, Sigma-Aldrich) was added to a crucible (30 mL) covered by a carbon cloth (Type A, 5.0 cm in diameter). The ferrocene-containing crucible was then heated to 450 °C. The loading of C/Hematite on the carbon cloth was ca. 1.0 mg cm− 2, which was assessed by evaluating the weight of the carbon cloth (CC) before and after the hematite coating. For comparison, the same amount of chemically synthesized hematite (denoted as Hematite) and hematite mixture with carbon powder (denoted as C–Hematite) was coated onto the surface of CC, as suggested by Lowy et al. [18]. The ratio of carbon powder to hematite was 1:5.7, which was the same as that in C/Hematite. Hematite was chemically synthesized following the method of Yen et al. [19]. 2.2. Formation of S. oneidensis biofilms on various electrodes S. oneidensis MR-1 bacterial culture suspension (1.0 mL) was firstly inoculated into 50 mL fresh LB broth (peptone, 10.0 g/L; yeast extract, 5.0 g/L; NaCl 1.0 g/L, pH 7.0), and incubated at 30 °C till the optical density (OD600) of the cell culture reached ~2.0. Then, the cells were harvested by centrifugation and washed 3 times with N2-purged phosphate buffer (Na2HPO4·12H2O, 11.4 g/L; NaH2PO4·2H2O, 2.8 g/L; KCl, 0.13 g/L; NH4Cl, 0.31 g/L, pH 7.0). The cell pellets were subsequently re-suspended in 50 mL electrolyte (phosphate buffer supplemented with 10 mM sodium lactate). The cell suspension was transferred into bioelectrochemical systems (BESs), and all working electrodes were poised at − 0.1 V vs. saturated calomel electrode (SCE). The BES (28 mL in volume) had a configuration as shown in Fig. S2. Bioelectrochemical experiments were carried out under multi-channel potentiostatic control (CHI1000C, China) utilizing a three-electrode arrangement with the as-prepared hematite electrodes as the working electrode (1.0 cm in diameter), a SCE reference electrode and a Pt net counter electrode. All tests were conducted in triplicate, and the mean values and typical curves are presented. 2.3. Electrochemical measurements Cyclic voltammetric (CV) measurements were performed with the biofilm-attached electrode as the working electrode under turnover conditions. Electrochemical impedance spectroscopy (EIS) was performed using an Autolab PGSTAT 302N potentiostat (Utrecht, the Netherlands) with NOVA software. The EIS of the biofilm-attached electrode was measured at the open circuit potential and in the frequency range of 1 × 106 to 0.5 Hz with a sinusoidal perturbation amplitude of 5 mV. The obtained Nyquist plots and circle fitting NOVA software were used to analyze the resistance of the bioelectrodes. Photoelectrochemical measurements of the hematites were conducted using a 300 W Xe lamp equipped with an interference filter with a band width of 10 nm as a light source, as suggested by Nakamura et al. [20]. Mott–Schottky plots were generated using the capacitance values derived from the EIS data fitting, from which the flat band potential and charge-carrier concentration were extracted as previously suggested [21]. The EIS measurements were also performed by a three-electrode configuration using Autolab PGSTAT as described above. A sinusoidal voltage perturbation with an amplitude of 5 mV and a frequency ranging from 1 × 105 to 1 Hz was superimposed on the bias voltage. The impedance was measured at bias voltages from 1.1 to 2.1 V vs. reversible hydrogen electrode (RHE). The RHE was converted from the SCE with the followed formulation:
ERHE ¼ ESCE þ 1:051 V ð1 M KOHÞ:
ð1Þ
All EIS measurements were performed in the dark. The Nyquist plot obtained from the EIS measurements was simulated using the NOVA software. The flat band potential and carrier density at the hematite/electrolyte interface can be obtained by the Mott–Schottky equation: 2
1=C ¼ ð2=e0 εε0 Nd Þ½ðE−EFB Þ−kT=e0
ð2Þ
where C is the specific capacitance (F/cm2), e0 is the electron charge, ε is the dielectric constant of hematite, ε0 is the permittivity of vacuum, Nd is the carrier density, E is the electrode applied potential, EFB is the flatband potential, and kT/e0 is a temperature-dependent correction term. 2.4. Analytical techniques The X-ray diffraction (XRD) patterns of hematites were recorded on an Empyrean diffractometer at room temperature, operating at 40 kV and 40 mA, using a Cu Kα radiation (λ = 0.15418 nm). The X-ray photoelectron spectra (XPS) were recorded on an ESCALAB 250 (Thermo Fisher Scientific) using monochromatic Al Ka X-rays (1486.69 eV) with an X-ray power of 150 W. Scanning electron microscopy (SEM) was used to confirm the morphologies of the prepared electrode and the biofilms as described previously [11]. Ferrous and ferric irons were monitored spectrophotometrically by the ferrozine assay according to previous methods [22]. All measurements were operated in duplicate and the typical image and curve were presented. 3. Results and discussion 3.1. Preparation and characterization of the C/Hematite-coated electrode Illustration of the experimental setup for the synthesis of the C/Hematite-coated electrode is found in Fig. S1A. The principle of the synthesis is that ferrocene was first melted and boiled in a crucible, then sublimed and decomposed to form hematite upon oxidation of oxygen as the temperature increased. As a result, a layer of red-colored film can be observed on the substrates covering the crucible. Using this method, Deng et al. has previously proposed the manipulation of
A
B
1 μm
1 μm
D
C
1 μm
1 μm
Fig. 1. SEM images of the blank CC (A), C/Hematite covered CC (B), Hematite covered CC (C) and C–Hematite covered CC (D, inset: SEM image in low resolution shows the presence of carbon particles as indicated by the arrows).
S. Zhou et al. / Bioelectrochemistry 102 (2015) 29–34
C/Hematite-covered conductive glasses, from which a high photocurrent was achieved for solar water splitting [23]. The enhanced photocurrent was attributed to the modified electronic structure of the C/Hematite with the existence of oxygen vacancies because of the presence of the carbon layer. However, it was noted that the hematite coating on the glasses was not homogeneous because of the inconsistent oxygen exposure. Herein, we covered a piece of carbon cloth (CC) on a crucible with a homogeneous layer of red-colored film (Fig. S1A). Carbon cloth is a type of carbon fiber fabric in a weave pattern with plenty of homogeneously distributed holes (Fig. S1), which ensures homogeneous delivery of oxygen for reaction with ferrocene vapor. As shown in Fig. 1A–B and Fig. S1B–E, the SEM images provide further evidence that C/Hematite was homogenously coated onto the CC. This method resulted in C/Hematite nanoparticles, which differ from the chemically synthesized rod-shaped hematite (Fig. 1C). The image of C–Hematite clearly shows the combination of hematite with carbon particles (Fig. 1D). Elemental mapping and Energy-dispersive X-ray spectroscopy (EDS) revealed that Fe, O and C were the main elements in the resultant hematite in which C was distributed into the hematite (Fig. 2A and B). XRD patterns of the resultant products from ferrocene exhibited similar signatures to those of hematite (Fig. 2C), indicating the successful synthesis of hematite from ferrocene. The observed reflections were indexed according to the rhombohedral structure and were consistent with the JCPDS data (card no. 33-0664). The observed sharp, intense reflections confirm the highly crystalline nature of the powders synthesized by heat treatment. XPS also revealed that Fe, O and C were the main elements in the C/Hematite (Fig. S3). The C1s XPS spectra showed a signal at approximately 284.8 eV, indicating the presence of C–C bonds in the C/Hematite. The binding energies for Fe2p3/2 and Fe2p1/2 are 711.2 and 724.6 eV, respectively, which are in excellent agreement with the reported value for Fe2O3 [24]. The O1s XPS spectra showed a signal at approximately 530.1 eV, which was also consistent with the reported value for Fe2O3 (530.1 eV for the original α-Fe2O3 and 530.3 eV for doped α-Fe2O3) [24]. The shoulder at higher binding energy (531.8 eV) could be attributed to O–C bonds.
31
3.2. Biocurrent generation enhanced by C/Hematite To evaluate the effect of C/Hematite on the interfacial EET of S. oneidensis, a BES was constructed with the fabricated electrodes as the working electrodes. S. oneidensis cells were first inoculated into the BESs, and all working electrodes were poised at − 0.1 V vs. SCE (Fig. S2, Supporting information). As shown in Fig. 3A, the microbial electric currents gradually increased with time, reflecting lactate oxidation by cells and electron transfer from the cells to the electrodes. The current from the bare CC electrode reached a maximum current density of 0.036 ± 0.005 mA cm−2, and a similar level using carbon papers was previously reported [25,26]. When the CC was coated with chemically synthesized hematite, the current reached a maximum of 0.060 ± 0.007 mA cm− 2. The current density further increased, reaching a maximum of 0.09 ± 0.006 mA cm−2 when the CC was coated with the C–Hematite mixture. Notably, after the C/Hematite coating, the current density dramatically increased to a maximum of 0.22 ± 0.01 mA cm−2, representing a nearly 6-fold increase compared to that of a bare CC electrode. A coulombic efficiency (CE) of 34 ± 3% was obtained in the BES with the C/Hematite electrode, which is higher than that of C–Hematite (19 ± 2%), Hematite (13 ± 2%), and CC (7 ± 1%) electrodes. The higher current densities and CE from the hematitecoated electrode are partially attributed to the higher affinity of the hematite-coated electrodes for bacterial cells compared to that of the bare CC electrode. The strengthened interaction between bacteria and C/hematite made the electrode attractive enough for the bacteria to transfer the electrons, which weakened alternative processes that did not contribute to current production and subsequently increased the CE. As shown in Fig. S4, more bacterial cells were observed on the hematite-coated CC than those on the bare CC. As for S. oneidensis MR-1, the outer membrane c-type cytochrome exhibited a specific binding to iron oxides [27]. This interaction not only enabled bacteria to insert electrons into the iron oxides [28] but also improved the bacteria/solid interfacial electron transfer [29]. Fig. S5A shows that the stable current output was achieved from the BES with the bacteriaattached C/Hematite electrode under the batch mode operation,
A
10 μm
Fig. 2. (A) Elemental mapping of C/Hematite: carbon (red), oxygen (yellow), and iron (green); (B) EDS analysis of C/Hematite; (C) XRD analysis of Hematite (red line) and C/Hematite (black line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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3.3. Electrochemical characteristics of the bioelectrodes The extracellular electron transfer capability of the S. oneidensis cells was further examined using CV scans. Fig. 3B shows the whole-cell CV recorded from various cell-attached bioelectrodes. The CV data of the bioelectrodes showed a pair of well-defined redox peaks in the potential range between −0.7 and 0 V vs. SCE, in which a midpoint potential (Em) was calculated according to Em = (Epc + Epa) / 2. An Em value of −0.160 V vs. SCE was obtained for the S. oneidensis cells attached to the CC electrode. This value is in accordance with the previously reported value for OM c-Cyts isolated from Shewanella and also Shewanella cells [32,33]. These results suggested that S. oneidensis cells expressed c-type cytochromes that are essential to the EET and could interact elecrochemically with external electrodes. To further understand the high current generation by the C/Hematite-coated CC, AC impedance was performed. Fig. 3C and D shows the typical Nyquist impedance spectra of bioelectrodes with the equivalent circuit model, where the independent axis is the real impedance (Z′ (ohm)) and the dependent axis is the imaginary impedance (−Z″ (ohm)). The parameter Rs represents the solution resistance, R1 is the resistance of the bioelectrodes, and R2 is the charge transfer resistance [34]. Constant phase elements (CPE1 and CPE2) are used to model non-ideal capacitors of the biofilm and double layer [34], respectively, defined as ZCPE = 1 / [Q(iω)n], where Q has a dimension of F sn − 1, and n is a dimensional parameter accounting for non-ideal behavior [35]. The fitting results are listed in Table 1. The Rs values for the BESs with various bioelectrodes were nearly identical because of the use of the same electrolyte. However, the different R1 values were observed due to the varied biomass loading and coating materials. The lowest R1 value was obtained from the C/Hematite-coated bioelectrode, likely resulting from the enhanced interaction between S. oneidensis cells and C/Hematite-coated electrode. The higher Q1 value of the C/Hematite-coated bioelectrode suggested the presence of the more active biofilm due to its properties as a charge storage body [36], and the higher Q2 value demonstrated the higher double layer capacitance of the C/Hematite than those of the other bioelectrodes [37]. In addition, the R2 values of the bioelectrodes were significantly varied. The R2 can serve as a measure of the electrocatalytic activity of the electrode because the R2 value is inversely proportional to the exchange current density of the reaction that takes place at each electrode [38]. An R2 of 14 Ω was achieved from the C/Hematite electrode, which is lower than that of the C–Hematite, Hematite and CC electrodes at 22, 36, and 60 Ω, respectively. The lowest R2 suggested the fastest electron transfer rate of the C/Hematite-coated bioelectrode. Fig. 3. Electrochemical characterizations of various electrodes (black line — C/Hematite, red line — C–Hematite, green line — Hematite, and blue line — CC): (A) Current generation by S. oneidensis with different electrodes poised at −0.1 V vs. SCE (the arrow indicates the addition of sodium lactate); (B) CVs of biofilm-based electrodes in the presence of substrate, scan rate = 10 mV s−1; (C) Nyquist curves of the EIS tests: black squares — C/ Hematite, red dots — C–Hematite, green triangles — Hematite, blue rhombs — CC, magenta lines — fitting curves (inset: the equivalent electrical circuit and the enlarged highfrequency region). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
suggesting the high stability of the C/Hematite electrode. It is well known that iron ions could be released from hematite in the presence of iron reducing bacteria [30], which positively affects the electron transfer rate of S. oneidensis [31]. As shown in Fig. S5B, release of iron ions from C/Hematite was also observed; however, only a very small amount of iron (total iron concentration of ~ 5 μM) was detectable in the electrolyte. With this small amount of iron, the current output was slightly affected. As shown in Fig. S5C, the maximum current density from the biofilm-attached CC electrode was 0.035 mA cm− 2, which slightly increased to 0.038 mA cm − 2 after the addition of 5 μM exogenous ferrous ions. The results suggested that the improved current generation from the C/Hematite electrode mainly resulted from the direct electron transfer facilitated by C/Hematite rather than the indirect electron transfer mediated by the released iron.
3.4. Conduction-band edge dependence of hematite stimulated EET To understand the role of carbon-coating on the electronic properties of hematite in electrolyte solution, EIS was conducted in the dark to determine the capacitance of hematites. The carrier density and flatband potential (EFB) at the hematite/electrolyte interface were evaluated by the Mott–Schottky equation. By analyzing the Mott–Schottky plots (Fig. 4), the EFB potentials were determined to be 0.37, 0.39, and 0.44 V vs. RHE for hematite, C–Hematite, and C/Hematite, respectively. The EFB potentials were consistent with the values previously reported for other hematites [21]. The charge-carrier concentrations were calculated to be 1.7 × 1019, 2.1 × 1019, and 3.7 × 1019 cm−3 for hematite,
Table 1 EIS fitting results for various bioelectrodes. Condition
Rs (Ω)
R1 (Ω)
Q1 × 10−4 (F sn − 1)
n1
Q2 × 10−6 (F sn − 1)
n2
R2 (Ω)
C/Hematite C–Hematite Hematite CC
6 6 7 5
189 261 387 545
9.3 5.5 3.4 2.4
0.80 0.80 0.90 0.80
4.69 2.89 2.36 1.88
0.80 0.80 0.79 0.70
14 22 36 60
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Fig. 4. Mott–Schottky plots of hematites collected at a scan rate of 10 mV/s (in 1 M KOH electrolyte at a frequency of 10 kHz), from which the flat band potential and charge-carrier concentration were extracted: C/Hematite (squares), C–Hematite (dots), and Hematite (triangles).
C–Hematite, and C/Hematite, respectively. With a higher EFB value and a larger charge-carrier concentration, the C/Hematite-coated electrode obtained better bioelectricity generation in the BES inoculated with S. oneidensis. Low conductivity is one of the major factors limiting the use of hematite in energy conversion applications, which is related to the charge-carrier concentrations and/or carrier mobility [39]. In comparison to unmodified hematite, the higher charge-carrier concentration demonstrated that higher electrical conductivity of C–Hematite and C/Hematite could be obtained, which benefit the hematitemediated electron transfer on the hematite–microbe interface. It is well known that S. oneidensis MR-1 applies a sophisticated protein network to transfer electrons from the cytoplasmic membrane across the periplasmic space and through the outer membrane to the extracellular cell surface, where a putative metal reductase stimulates terminal electron transfer directly to solid acceptors [40,41]. The terminal metal reductase is confirmed to be composed of outer membrane, decaheme c-type cytochromes. In particular, two of these OM c-Cyts, MtrC (locus tag SO1778; also known as OmcB) and OmcA (locus tag SO1779), have been shown to play an important role in direct electron transfer to iron oxides in the case of S. oneidensis [27]. However, the matched energy level of terminal OM c-Cyts with the conductionband edge (ECB) of iron oxides was a requirement for the greatly stimulated EET [20]. Herein, the ECB values of the resulting iron oxides were evaluated to be approximately − 0.22, − 0.19, and − 0.17 V (vs. SCE) for hematite, C–Hematite and C/Hematite, respectively, from the onset potential of the anodic photocurrents (Fig. S6). It was found that the OM c-Cyts of Shewanella cells had a midpoint potential (Em = −0.160 V vs. SCE) close to the conduction-band edge of C/Hematite (ECB = −0.17 V vs. SCE), whereas a large energy separation existed for Hematite (ECB = − 0.22 V vs. SCE) and C–Hematite
Scheme 1. Conduction-band energy-level (ECB) diagram of the hematites with respect to the mid-point potential (Em) of outer-membrane c-type cytochromes for live S. oneidensis MR-1 cells.
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(ECB = −0.19 V vs. SCE) (Scheme 1). The better matched energy levels of the OM c-Cyts with the ECB of C/Hematite could contribute to the higher current generation of the C/Hematite-coated electrode compared to other hematite-coated electrodes. Indeed, several methods have been verified to design the electrochemical properties of the hematite for achieving better physical and chemical activities [35,42]. One effective method is to adjust the component of the hematite by using doping strategies [35]. As previously reported [43], the incorporation of “impurities” into hematite has two main effects: (i) the electronic band edges are modified; and (ii) the incorporation of some “impurities” increases the unit cell volume, which may also affect the electrical conductivity by changing the hopping probabilities. In this respect, the shifting of the ECB of the C/Hematite could be attributed to the incorporation of carbon into the hematite. However, improved electron transfer can also be achieved by dispersing fine particles of iron oxide onto the surface of the conductor [17,44]. For the same reason, carbon particles acted as the conductor for communicating the hematite colloids in the C–Hematite, leading to the shifted ECB and the accelerated EET compared to the non-modified hematite. 4. Conclusion In this study, carbon-coated hematite was proven to be an effective modification for high-performance electrodes with high biocurrent generation. The substantial improvement was attributed to the advantages of carbon–hematite coupling, which changed the electrical properties of hematite for electron transfer, resulting in an improved interaction between the microbial cells and the electrodes. Because the C/Hematite can be readily decorated onto electrode substrates through an evaporation method, C/Hematite may be a promising material for constructing highly efficient electrodes for BESs. Acknowledgments This study was supported jointly by the National Natural Science Foundation of China (Nos. 21277035 and 4122006). Appendix A. Supplementary data SEM and XPS analysis of electrodes and electrochemical properties of all hematite-coated electrodes (Fig. S1–6). Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j. bioelechem.2014.11.005. References [1] B.E. Logan, B. Hamelers, R. Rozendal, U. Schröder, J. Keller, S. Freguia, P. Aelterman, W. Verstraete, K. Rabaey, Microbial fuel cells: methodology and technology, Environ. Sci. Technol. 40 (2006) 5181–5192. [2] W.W. Li, H.Q. Yu, Z. He, Towards sustainable wastewater treatment by using microbial fuel cells-centered technologies, Energy Environ. Sci. 7 (2014) 911–924. [3] D.R. Lovley, Live wires: direct extracellular electron exchange for bioenergy and the bioremediation of energy-related contamination, Energy Environ. Sci. 4 (2011) 4896–4906. [4] B.H. Kim, H.J. Kim, M.S. Hyun, D.H. Park, Direct electrode reaction of Fe (III)-reducing bacterium, Shewanella putrefaciens, J. Microbiol. Biotechnol. 9 (1999) 127–131. [5] D.R. Bond, D.E. Holmes, L.M. Tender, D.R. Lovley, Electrode-reducing microorganisms that harvest energy from marine sediments, Science 295 (2002) 483–485. [6] K. Rabaey, N. Boon, M. Höfte, W. Verstraete, Microbial phenazine production enhances electron transfer in biofuel cells, Environ. Sci. Technol. 39 (2005) 3401–3408. [7] A.P. Borole, G. Reguera, B. Ringeisen, Z.W. Wang, Y.J. Feng, B.H. Kim, Electroactive biofilms: current status and future research needs, Energy Environ. Sci. 4 (2011) 4813–4834. [8] L. Peng, S.J. You, J.Y. Wang, Carbon nanotubes as electrode modifier promoting direct electron transfer from Shewanella oneidensis, Biosens. Bioelectron. 25 (2010) 1248–1251. [9] Y. Qiao, S.J. Bao, C.M. Li, X.Q. Cui, Z.S. Lu, J. Guo, Nanostructured polyaniline/titanium dioxide composite anode for microbial fuel cells, ACS Nano 2 (2007) 113–119. [10] X. Xie, L. Hu, M. Pasta, G.F. Wells, D. Kong, C.S. Criddle, Y. Cui, Three-dimensional carbon nanotube-textile anode for high-performance microbial fuel cells, Nano Lett. 11 (2010) 291–296.
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